Electrolyte electroreflectance spectroscopy (EER) and electrode admittance measurements were used to investigate the potential distribution across the GaAselectrolyte and Ga1-xAlxAselectrolyte interface. The experimental EER spectra of epitaxial n+-GaAs and Ga1-xAlxAs (0 < x greater-than-or-equal-to 0.30) obtained under conditions of photodissolution were compared with theoretical low and intermediate field spectra. A qualitative comparison enabled the potential distribution to be determined and demonstrated that all the semiconductor electrolyte junctions behave in a non-ideal manner. Extensive Fermi level pinning was observed near flatband, and this was attributed to the formation of adlayers by reduction of soluble Ga(III) and As(III) solution species. EER spectra measured with a low etch pit density (< 10(4) cm-2) epitaxial GaAs sample were quite different, indicating that the non-ideal behaviour observed for other samples is due at least in part to surface defects. The degree of Fermi level pinning was found to depend on semiconductor composition; high aluminum content Ga1-xAlxAs samples behaved most ideally.